Tetramethylthiourea (TMTU) as an alternative redox mediator for electrochromic devices

Iodide/triiodide (I−/I3−) redox electrolyte in hybrid electrochromic (EC) devices provides high charge capacity and thereby a high optical modulation can be achieved. However, its corrosive nature and absorption in UV and in blue range of light forming an unpleasant yellowish coloration limits smart window application. In this regard, tetramethylthiourea/tetramethylformaminium disulphide (TMTU/TMFDS2+), 2,2,6,6-tetramethyl-1-piperidinyl-oxy (TEMPO)/TEMPO+ and potassium ferrocyanide/potassium ferricyanide (K4[Fe(CN)6]/K3[Fe(CN)6]) are studied as alternative redox couples. Their characterisation is performed considering electrochemical and optical properties. The redox potential is evaluated by cyclic voltammetry. The charge transfer resistance (Rct) is determined by electrochemical impedance spectroscopy on sputtered and sol-gel tungsten trioxide (WO3), sol-gel titanium oxide (TiO2), fluorine doped tin oxide (F:SnO2) as well as on sputtered platinum (Pt). TMTU/TMFDS2+ has demonstrated an appropriate redox potential as well as a high charge transfer resistance on oxides and low on Pt layers for high bias potentials. Furthermore, the kinetics of the change in transmission of the EC devices is compared considering the loss current. The EC device comprising TMTU/TMFDS2+ exhibits excellent optical modulation in visual transmission from 71 down to 14% at −795 mV within 5 min.